The invention relates to digital watermarking, and more particularly relates to watermark detection in multimedia content (e.g., still image, video and audio signals).
The reader is presumed to be familiar with digital watermarking technology. See, e.g., co-pending application Ser. No. 09/503,881, filed Feb. 14, 2000, entitled Watermark Reader and Embedder, the disclosure of which is incorporated by reference.
One objective of watermark detectors is to reject unmarked signals (e.g., image, audio, video signals) at the earliest possible stage of detection. The detector may conclude that a signal is unmarked based on quantitative evidence of the watermark (or lack thereof) in a signal suspected of having a watermark. The signal might be an unmarked component of a marked signal, or simply an unmarked signal. Also, in some cases, the signal, though previously marked, may appear to be unmarked due to removal or degradation of the watermark. By accurately identifying an unmarked signal at an early stage, the detector can avoid unnecessary processing. Also, the apparent absence of a watermark may trigger some action (or prevent an action) such as providing output indicating that the signal has been tampered with or controlling processing of the signal (e.g., preventing copying, playing or recording in copy protection applications).
A related objective of a watermark detector is measuring the strength of a watermark signal. Based on the watermark strength, the detector can assess whether a suspect signal has a valid watermark, and the extent to which a signal has been transformed. The detector can also determine the likelihood that a suspect signal includes a valid watermark or recoverable watermark message. Such an evaluation helps the detector allocate its processing resources on portions of the suspect signal that are likely to contain a valid watermark or recoverable watermark message.
The cited application describes a variety of techniques for detecting a watermark. Some of these techniques correlate attributes of a watermark signal with a signal suspected of containing a watermark. By measuring the extent of correlation, a watermark detector assesses whether a watermark is present, and in some cases, determines its orientation in the suspect signal. Related techniques detect a watermark signal by at least partially decoding a message from the suspect signal and then comparing attributes of the message with expected attributes to assess the likelihood that a watermark signal is present. These and other techniques may be used to compute a detection value that quantifies the likelihood that the suspect signal has a watermark.
This disclosure describes methods of using detection values ascertained from signals suspected of being watermarked to control the detection process. The detection values may be used to reject unmarked signals. In addition, they may be used to refine the detection process by focusing the detector on signals or portions of signals that are likely to contain a watermark and/or a recoverable watermark message. Each portion of a suspect signal may be defined by an orientation parameter (or set of parameters like rotation, scale, origin, shear, differential scale, etc.). Also, each portion may represent different orientations of the suspect signal, or a component of the signal.
This disclosure also describes a method for using two or more detection metrics to control the detection process. The multiple metrics could be derived from independent measurements in multiple stages or could be different features of the same measurement. Each detection metric evaluates detection values to control detection actions. One type of detection metric is a screen used to evaluate suspect signals or portions of a suspect signal for the presence of a watermark. Each stage evaluates detection values to assess whether a suspect signal, or portion of it, is marked.
This disclosure also describes a method for using absolute and relative detection measures to assess whether a suspect signal is marked. An absolute measure of detection represents quantitative evidence of a watermark signal in a suspect signal, and is usually evaluated independently from other detection values. A relative measure is based on the relative values of two or more detection values, which may be relative or absolute measures. A relative measure may be implemented by computing absolute detection values for different portions of a suspect signal and then computing a relative detection value as a function of the absolute detection values.
Both absolute and relative detection values may be evaluated relative to desired limits or thresholds to determine an appropriate action. One action is to reject the candidate signal associated with the detection value as being unmarked. Another action is to use the detection values to direct further actions of the detector. One advantage of using both absolute and relative detection values is that they usually contain complementary information. This complementary information helps in improving the watermark screening and detection process.
In one implementation, a detector computes detection values for different orientation parameter candidates, sorts the detection values in terms of likelihood of representing a valid watermark, and then takes a ratio of a top detection value relative to one or more lesser detection values. The orientation parameter candidates define an approximate orientation and/or location of a watermark in a suspect signal, and as such are associated with a portion of a suspect signal.
The invention provides a method of analyzing a signal suspected of being embedded with an auxiliary signal. This method computes detection values from a suspect signal. Each detection value corresponds to a measure of an auxiliary signal embedded in the suspect signal. The auxiliary signal being detected comprises a set of attributes embedded at locations within the suspect signal, and each detection value is a measure of the set of attributes. The method computes a relative detection value based on an analysis of two or more of the detection values relative to each other, and controls detector action based on the relative detection value. This detector action may include distinguishing signals that are marked with an auxiliary signal from those that are not. It may also include determining whether the suspect signal is authentic based on the detection values associated with an embedded auxiliary signal.
Another aspect of the invention is a method of detecting an auxiliary signal embedded in a suspect signal. The embedded auxiliary signal has a set of attributes embedded at locations within the suspect signal. The method computes detection values for two or more auxiliary signal candidates in the suspect signal. It evaluates a detection metric based on the detection values, and controls a detector action based on the detection metric. The metric provides a measure of an embedded auxiliary signal that adapts to the suspect signal according to detection values computed from the suspect signal. The detection values are each computed as a measure of the set of attributes of the auxiliary signal.
Another aspect of the invention is a detector for analyzing suspect signals having embedded auxiliary signals. The detector has a first screening stage for classifying a suspect signal based on a first detection metric that measures a set of attributes of an auxiliary embedded signal. It also has at least a second screening stage for classifying the suspect signal based on a second detection metric that measures a set of attributes of an auxiliary embedded signal. The first and second stages form a multistage classifier for determining whether a suspect signal is authentic based on measures derived from an embedded auxiliary signal.